Multi layer difusion coatings and method of applying the same



Jan. 14, 1964 FAQ JEN c o 3,117,846

II/IULTI LAYER DIFFUSION COATINGS AND METHOD OF APPLYING THE SAME Filed Jan. 28, 1960 2 Sheets-Sheet l MOLYBDENUM CHROMIUM REACTION BASE MIXTURE BASE PACKED IN REACTION MIXTURE COOL RAPIDLY CLEAN COATED MOLYBDENUM-SILICON BASE REACTION MIXTURE PACK IN REACTION MIXTURE COOL RAPIDLY CLEAN I FIG.I

INVENTOR. PAO JEN CHAO ATTORNEY Jan. 14, 1964 FAQ JEN CHAO 3,117,846

MULTI LAYER DIFFUSION COATINGS AND METHOD OF APPLYING THE Filed Jan. 28, 1960 2 Sheets-Sheet 2 IN VEN7UR. PAO JEN CHAO A TTORNEYS United States Patent 3,117,846 MULTI LAYER DIFFUSION CUATINGS AND METHDD 0F APPLYING THE SAME Pao Jen Chao, Scottsville, N.Y., assignor to Pfaudler Permutit Inc, Rochester, N.Y., a corporation of New York Filed Jan. 2%, 1960, Ser. N 5,240 6 Claims. (Cl. 29195) This invention relates to diffusion coatings for metallic objects and methods for applying the same, and more particularly to mult-i-layer diffusion coatings for refractory metals, one object being the provision of a more satisfactory coating of this nature.

Refractory metals have come into increasing use as structural materials, mainly because of their ability to maintain their strength at elevated temperatures. Modern technology requires many parts which are subjected to very high temperatures such as, for example, service as components of gas turbines, rocket engines and the leading edges of missiles or space vehicles designed for flight in the atmosphere at high speeds. However, the surface of many of these metals are vulnerable to oxidation, corrosion or erosion and other deleterious effects at elevated temperatures or when exposed to rapidly moving gases. For this reason, the provision of a protective coating is often necessary.

lvlolybdenum and its alloys are one of the few structural materials available which maintain a high strength at these elevated temperatures, and therefore, these materials have come into increasing use for the fabrication of components of this nature. However, these metals and alloys have one common shortcoming; that is, their poor resistance to oxidation at these high temperatures. For this reason, it has been necessary to apply oxidation resistant coatings to molybdenum arid its alloys wherever the service encountered includes exposure to air or oxygen at elevated temperatures. The provision of a more satisfactory coating of this nature which will resist oxidation at high temperatures is, therefore, another object of this invention.

Molybdenum and its alloys also possess certain characteristics which limit the types of oxidation resistant coatings that heretofore have been possible to apply for increas ng the oxidation resistance at elevated temperatures. These metals have desirable strength and ductility when in a certain metallurgical condition but other metallurgical conditions thereof are undesirably brittle. When molybdenum is maintained at a high temp-mature for a prolonged period of time, particularly in a vacuum, the microstructure of the molybdenum changes from a line grained, ductile structure to a coarse grained structure which results in enbrittlement of the metal and its alloys. This characteristic of the alloy makes it undesirable to apply any coating thereto which requires the maintenance of high temperatures for a lengthy period of time since grain growth may start during the application of the coating which renders the part less suitable for later service at high temperatures. in other words, the more nearly the structure of the molybdenum can be preserved in its in -al fine grain state during the coating process, the longer the useful high tem erature life of the metal will be in service. For this reason, another object of this invention is the provision of a high temperature oxidation resistant coating on molybdenum and its tdloys which may be applied at a relatively low temperature and/ or during a relatively short time.

High temperature coatings of the ceramic type have been applied to alloys of this nature in the past. However, in order to be efiective, such coatings must maintain their integrity at the operational temperatures to artists Patented Earn. 1 193A "hich the alloy is subjected; that is, temperatures up to and in the range of 2500" to 3000 P. This means that the ceramic coating must be fused to the base metal at temperatures equal to or exceeding the highest temperatures encountered by the alloy during service, and this requires the subjection of the alloy to these temperatures for the period of time necessary to fuse the coating to form a coherent continuous film. Further, since such coatings are usually applied in several layers, the alloy must be repeatedly subjected to these temperatures in order to provide a finished coating. It has been found that this treatment will cause at least incipient recrystallization and grain growth in the molybdenum alloy, so that the finished product is either brittle or incipiently so. in addition, mwhanioal properties of ceramic coatings are undes rable.

Other coating techniques have been attempted for providing molybdenum alloys with the desired oxidation resistant coating. One of the most common types of coatings used for this purpose is the application of chromium by the cementation process. in this process, the molybdenum is subjected to chromium or a reactive chromium compound at elevated temperatures, and the chromium is deposited on the surface of the molybdenum. This chromium then diffuses inwardly building up a layer of chromium rich molybdenum alloys at the surface of the object. However, these chromized coatings offer only very limited resistance to high temperature oxidation in excess of 2208 F., and therefore have not been found satisfactory for service as high as 3080" F. Further, because of thermodynamics involved in the conventional chrorm'zing processes for molybdenum, more lengthy treatment by the chromizing processes will not afford a thicker or more corrosion resistant coating since unlimited material solid state solubility of chromium and molybdenum causes the chromium to diffuse into the molybdenum substrate at a rate equal to or higher tha. the deposition rate. This means that even prolonged treatient by the conventional chromizing processes will not result in a surface surficiently rich in chromium to resist oxidation at the desired elevated temperatures. In addition, this mutual solubility leads to an inward dittusion of the chromium during the high temperature service and thus the chromium treated surface becomes progressively poorer in chromium during the service of the coated part, ultimately ending in failure by oxidation.

Other high temperature coatings for the protection of molybdenum and its alloys at high temperatures have been attempted. ()ne of the most widely used is the use of the application of silicon by the diffusion process for the production of an oxidation resistant surface of molybdenum silicides. However, it has been found that when silicon is deposited onto molybdenum, it also ditfuses inwardly forming a series of molybdenum-silicon compounds of various compositions. For example, a molybdenum or molybdenum alloy partly coated with silicon by the diliusion process consists of a pure molybdenum or molybdenum alloy core with a layer of trimolybdenum silicide covered by a layer of tri-molybdenum di-silicide which in turn is covered by another layer of molybdenum di-silicide. The surface layer generally consists of molybdenum di-siiicide which is highly resistant to oxidation at elevated temperatures. However, the intermediate layers of id-molybdenum silicide and trimolybdenurn di-s' icide are unstable and brittle, and have a coefficient of thermal expansion which differs considerably from that of the molybdenum alloy and the surface layer of molybdenum di-s icide. This chemical instability and difiering coefiicient of expansion makes the coating sensitive to thermal shock and renders such coatings unsatisfactory for high tem erature use. For

this reason, the provision of a coating consisting principally of molybdenum di-silicide, free from the intermediate layers of other molybdenum-silicon compounds, is another object of this invention.

It has been found that the formation of these intermediate molybdenum-silicon compounds described above can, in large measure, be prevented by a multi-cycle deposition process wherein a thin barrier layer of a heat resistant alloy is applied to the surface of the molybdenum or the molybdenum alloy object, and then a second layer of desired molybdenum di-silicide is deposited on the surface thereof. If the composition of the intermediate or barrier layer is properly adjusted, the amount of molybdenum diffusing outwardly therethrough into the surface layer of the desired molybdenum silicon compound can be controlled and if the composition of the molybdenum silicon alloy deposited is properly controlled, the final result is a thick, coherent surface coating consisting substantially of the desired molybdenum-silicon compound substantially uncontaminated by other undesired molybdenum-silicon compounds described above. For this reason, the provision of such a multi-cycle process for the diffusion coating of molybdenum and its alloys, resulting in a surface coating of the desired molybdenum silicon alloy for protection of the molybdenum from high temperature oxidation, is another object of this invention.

Other objects of this invention include the provision of a coating process which may be carried out during a short period of time and at a relatively low temperature which will prevent the recrystallization and consequent enbrittlement of the molybdenum alloy.

Other objects include the provision of an oxidation resistant coating for molybdenum and its alloys which will be resistant to oxidation at temperatures up to and including 3000" F.

Another object of this invention is the production of a strong diffusion bond between the molybdenum substrate and the chromium-molybdenum intermediate layer.

Another object of this invention is the production of a chemical bond assisted by diffusion between the chromium-molybdenum intermediate layer and the molybdenum disilicide outer layer.

Another object is the co-deposition of molybdenum and silicon, the ratio being one atom of molybdenum to two atoms of silicon to form MoSi by controlling the individual vapor pressure of volatile ingredients of molybdenum and silicon.

Other objects include the provision of a more satisfactory economical and stable oxidation resistant high temperature coating for molybdenum which may be applied by means of relatively simple apparatus.

Other objects and advantages-of this invention will be particularly set forth in the claims and will be apparent from the following description, when taken in connection with the accompanying FIG. 1 which comprises a simplified flow diagram of the process of this invention, and FIG. 2 a photomicrograph of a cross-section of molybdenum alloy coated by the said process at a magnification of approximately 750 diameters.

During the practice of the processes embodying the present invention, the desirable end product is a substrate of fine grain molybdenum or molybdenum alloy covered with a continuous coherent coating of molybdenum disilicide. This latter compound is not only resistant to oxidation at temperatures in the range of 2500 to 3000 F., but has a coefficient of thermal expansion similar to that of the molybdenum itself so that such a coating is effective in protecting the molybdenum throughout the entire range of high temperature service up to 3000 F. However, where silicon is deposited on molybdenum, the difiusivity of these elements and their chemical afiinity result in a series of molybdenum-silicon compounds at the interface having varying compositions ranging from pure molybdenum in the core to the molybdenum di-silicide at the surface. It is these intermediate layers of unstable molybdenum silicon compounds having compositions intermediate to pure molybdenum and molybdenum di-silicide that cause the failure of the coating. Moreover, further diffusion of molybdenum and silicon takes place during the high temperature service of the coated part so that even though the coating were substantially pure molybdenum di-silicide on a substrate of pure molybdenum, these intermediate compounds would be formed by the outward diffusion of molybdenum and the inward diffusion of silicon during the high temperature service, resulting in these undesirable intermediate layers which are chemically unstable at high temperatures and which have thermal expansion coefficients differing markedly from both molybdenum and molybdenum di-silicide. Thus, when a change in temperature occurs, these coatings tend to change dimensions differentially and failure of the coating and exposure of the molybdenum to oxidation may occur.

The principle of this invention resides in the provision of an intermediate layer between the molybdenum substrate and the molybdenum di-silicide outer coating which has properties compatible with both the substrate and the coating, and which has a sufficiently low diffusivity of molybdenum so that it acts as a barrier for the outward diffusion of molybdenum to the coating, thereby preventing the undesirable change in composition and the formation of intermediate molybdenum silicon compounds during the coating stage and later high temperature service.

In order to have the intermediate or barrier layer effective for this purpose, it must consist in a large part of metals other than molybdenum and silicon. Further, in order to have the coating serviceable at the desired high temperatures, these other metals must be refractory; that is they must be able to withstand the high temperatures which molybdenum and molybdenum alloys encounter in service. They should also preferably be resistant to oxidation so that they aiford additional protection to the molybdenum substrate during service. While many of the refractory metals such as chromium, tantalum, cobalt, tungsten, Zirconium and others might be satisfactory, if properly adjusted, from a practical point of view, chromium is the most satisfactory metal for this particular application. Not only has chromium itself been used for an oxidation resistant coating, which has provided efiective at somewhat lower temperatures that the molybdenum disilicide coatings, but the chromium molybdenum alloys are compatible with the molybdenum di-silicide coating surface, arid chromium itself is substantially resistant to oxidation at elevated temperatures. It has been found satisfactory to apply a diffusion type of coating of chromium to the molybdenum substrate. The chromium diffuses inwardly into the molybdenum to form a coating of molybdenum-chromium alloy which in itself affords substantial protection to the molybdenum substrate. When molybdenum and silicon are applied by the reduction deposition diffusion mechanism to the surface of this: coating at a properly controlled rate, only the formation of molybdenum di-silicides takes place. A much smaller amount of molybdenum diffuses out xardly from the coating that would be the case if this coating were applied on pure molybdenum substrate, thereby avoiding the formation of undesirable intermediate compounds. The process therefore consists essentially in the application of a barrier layer combined with the co-deposition of molybdenum and silicon over this barrier layer rather than the application of pure silicon has heretofore been the practice.

This invention can best be described by reference to the following examples which are intended to be taken in an illustrative and not in a limiting sense.

EXAMPLE I T .e object to be coated is embedded in a powder hav- The object is carefully embedded in the above mixture and is then sealed in a reaction chamber. This chamber is then heated to a temperature of between 1750 to 2060" F. for a period from one-half to five hours depending on the depth of coating desired to allow the chromium to deposit on the surface of the molybdenum substrate and diffuse inwardly. This chamber is then cooled rapidly to room temperature (within a period of one to ive minutes, depending on the size of the reaction chamber) in order to prevent, as far as possible, any grain growth in the molybdenum substrate. The reaction chamber is then opened, and the coated molybdenum object removed from. the packing powder and cleaned by any suitable means such as brushing, sandblasting or the like to remove any adhering particles of the packing material.

The molybdenum object which has been pre-coated as described above is then embedded in a mixture having the following composition.

Percent Molybdenum powder 5 to Silicon powder 10 to Ammonium halide 2 to 8 Inert filler materials 50 to 80 The part is carefully packed in the powder having the above composition, and is placed in a reaction chamber which is carefully sealed. This chamber is then heated to a temperature from approximately 1990" to 2100 F. for a period of from one-half to ten hours, depending on the size of the part and the thickness of the coating desired. The coating mixture decomposes and deposits a layer of molybdenum di-silicide on the pre-coated surface, forming a continuous adherent coating of molybdenum disilicide over the entire surface of the part. At the end of the coating cycle the reaction chamber is cooled rapidly to room temperature, as described above for preventing, as far as possible, any change in the grain size of the molybdenum by exposure to high temperatures for a period longer than necessary.

It een found that the above procedure produces a reel Wing a microstructure which is substantially unchanged; that is, that practically no enbrittlernent at all was observed on parts coated by the above .c The entire surface of molybdenum is completely protected by the coherent coat of molybdenum disilicide. These parts resist elevated temperature exposures for extended periods as shown in Table I below.

EXAMPLE H first tho-r0 ghly cleaned and then suspended in an t reaction c ber, and the chamber is carefully A s reaction chamber is evacuated and the chamber is then preferably purged with argon in order to remove traces of ox gen. This argon isthen replaced by hydrogen. chamber is then heated to a ten perature from approximately 1450 to 2200" F. for a period ranging from one half to ten hours, depending on the size of the part arid the depth of coating IBilt During this period, the c amber is fed v r a gaseous m xture of a chi-on rous halide and pure dry hydrogen ferably in the ratio of approxhnately l to 2, although Lia can be va ied somewhat. The pressure within this reaction cham' er is preferably reduced below atmospheric, and is preferably ned at a pressure such as 18* atmospheres. The exhaust vapor-gas mixture may be led 6 to a trap in a condenser for recovering the unused chromous halide.

After the process has been carried on for the required length of time, the chamber is rapidly cooled to room temperature to avoid any unnecessary exposure of the molybdenum part to elevated temperatures. The chamber is then opened, and the coated part is removed and is then ready for the second step in the coating process.

The molybdenum silicide surface coating is applied to the object pre-coated as described above. The pre-coated object from the previous cycle is suspended in an airtight reaction chamber, and the chamber is carefully sealed. The air in this reaction chamber is then evacuated, and the chamber is filled with argon and then a mixture of hydrogen and argon is heated to a temperature ranging from approximately 1450 to 2006" F. for this process. The chamber is then fed with molybdenum pentahalide in vapor phase, and silicon tetrahalide and pure hydrogen gas. The flow rates of these three reactants is preferably maintained in the ratio of approximately 1 to 2 to 7, although this ratio can be varied, if desired.

The pressure within this reaction chamber is preferably kept well below atmospheric and the exhaust vapor gas mixture is led to a trap or a condenser to recover the unused molybdenum and silicon compound.

This heating is continued for a period ranging from one-quarter hour to five hours, depending on the thickness of the surface coating that is desired, which in turn is governed by the rigor of the service to which the part is to be subjected. After the deposition is complete, the reaction chamber is preferably cooled to room temperature at a rapid rate (during a period of from one to five minutes depending on the size or the reaction chamber) to prevent the undue exposure of the molybdenum part to elevated temperatures for a period longer than necessary.

The reactions which occur during the rare-coating cycle are as follows:

CO NII2 2 C0 N2 2112 (to replace air) NHiOl N113 H01 heat 21101 Hz C12 Cr+ 012 CrClz 2M0 561: 2M0Cl thermal Cr Mo solid state Solution diflusion CrCl +ll Cr+ Zl-lCl Cr Moe Solid State Solution (Jo-Deposition Cycle When the processes have been carried out as described above, either by the pacldn" process or the vapor deposition diffusion process, a two-layer coating results as clearly shown in the photomicrograph. The composition of the substrate 1 is substantially pure molybdenum or molybdenum alloy; the composition of the intermediate or barrier layer 2 is substantially chromium-molybdenum alloy of varying compositions; and the composition of the upper or protective layer 3 is substantially pure molybdenum di-silicide uncontaminated by other molybdenumsilicon compounds. Thus, the processes comprising this invention allows the deposition of a surface layer of molybdenum-silicon alloy of any desired composition. This is rendered possible by protecting this layer during deposition and subsequent service from contamination by molybdenum difiusing outwardly from the molybdenum substrate. A coating of any composition could be similarly placed on the surface of the barrier layer simply by controlling the composition of the reactant in the second or co-deposition phase of the process, and this invention is not intended to be limited to any particular composition although it has been described in terms of a molybdenum di-silicide coating.

The results of the processes described above clearly set forth in Table I reproduced below.

said molybdenum substrate by the diffusion process, and applying a coat of molybdenum di-silicide by co-deposition by the diffusion process on the surface of said chromium coated molybdenum substrate, said chromium coating being sufficiently thick to prevent the diffusion of said molybdenum substrate into said molybdenum disilicide.

2. The process as claimed in claim 1 wherein the coated substrate is rapidly cooled after the coating processes have been completed.

3. The process of applying a high temperature oxidation resistant coating to a molybdenum object comprising the steps of packing said object in a mixture of metallic chromium powder, a volatile halide, urea and inert filler material in a sealed chamber, heating the chamber to a temperature of between 1750 and 2060 Fahrenheit for a period ranging from one-half hour to five hours, cooling said chamber rapidly to room temperature, removing said object, and packing the same in a second mixture comprising a mixture of metallic molybdenum and silicon powders, a volatile halide, and an inert filler material, and sealing the same in a closed chamber, heating said chamber to a temperature from approximately 1900 to 2100 Fahrenheit from a period of from one-half hour to ten TABLE I.DATA OF TEST ON OXIDATION RESISTANCE OF COATING ON VARIOUS MOLYBDENULLI SUBSTRA'IES Thickness Thickness Thermal Shock of Substrate, of Coating, Oxidation Resistance Resistance Ductility Remarks Mils Mils 10" 2.6 2,600 F., 2hrs. Air cooled Substrate was brittle in as received condition. 220 2.6 2,600 F., min do Bcnt no failure Substate was ductile in as received con ition. 220 2.6 2,650 F.,30 min. (bent) "do Flfrrtlher bent to no at ure. 2O 2. 6 2,650 F., 30 mi n Cold air blasting Coating spalled slightly at Substrate was ductile after test. bend after air blasting. -30 2.6 3,000 F., 20 min. (blistcrcd do -t Coating slightly deterior- Substrate was ductile in as received slightly). atcd at bend after cold condition. air blasting. 40 2.6 3,000 F., 15min--- do Substrate was brittle in as received I condition. 40 2.6 3,0o0 F., 30 min. (blistered) dc 30 2.8 2,800 F., 10 hrs. (intermit- Bent 60 after oxidation test, Substrate was ductile in as received nt) no failure. condition. No cracks or other failure found in coating after tests.

This table shows the results of preliminary oxyacetylene tests on samples of commercial pure molybdenum coated as described above with a chromium-molybdenum intermediate barrier layer and a molybdenum di-silicide surface layer. These parts were heated to the temperatures designated in column 3, for the period indicated therein. These parts were then cooled as described in column 4, and others were then bent through various angles to test the ductility of the specimen. It is to be noted that none of the samples except the last two showed any signs of surface oxidation.

It will thus be seen that this invention contains its desired objects in providing a means for coating a metallic substrate with a surface coating of any desired composition, uncontaminated by intermediate layers caused by the mutual diffusion of the coating and the substrate. This process provides a means for coating metal parts of any desired composition by the diffusion process with any desired coating. While this invention has been described in terms of processes for coating molybdenum with molybdenum di-silicide, it is apparent that by selection of the proper barrier layers, that any desired metal could be coated with any desired coating composition using the process herein disclosed.

While I have shown and described the preferred form of mechanism of my invention it will be apparent that various modifications and changes may be made therein, particularly in the form and relation of parts, without departing from the spirit of my invention as set forth in ;prising the steps of applying .a coating of chromium to hours, and then cooling said chamber rapidly to room temperature. I

4. The process of applying a high temperature oxidation resistant coating to a molybdenum object comprising the steps of placing said object in a sealed reaction chamber, evacuating the air in said chamber, heating said object to a temperature of between 1450 and 2200 Fahrenheit for a period ranging from one-half to ten hours while subjecting it to a vapor comprising a chromous halide and hydrogen and then subjecting said object to a second vapor comprising hydrogen, a molybdenum halide and a silicon halide at a temperature of between l450 and 20-00 Fahrenheit for a period ranging from onefourth to five hours.

5. A process for applying a diffusion bonded coating to a substrate selected from the group consisting of tungsten, molybdenum, niobium and tantalum and their alloys, comprising the steps of applying an intermediate barrier coating consisting substantially of chromium to said substrate by the diffusion process and applying a second coating and consisting substantially of molybdenum disilicide over said intermediate barrier coating by the diffusion process, said intermediate coating being of sufficient thickness to substantially prevent the interdiilusion of the components of said substrate and said second coating to maintain said second coating at substantially original composition.

6. A metallic structure comprising, in combination, a base comprising one of the metals selected from the group consisting of molybdenum and molybdenum alloys, an intermediate diifusion barrier layer consisting essentially of chromium applied to said base by the difiusion process, and an oxidation resistant coating consisting substantially oi molybdenum disilicide applied on said inter- ":3 mecliate barrier 1ayer, sai barrier layer bsing of sufficient thickness to prevent the molybdenum of said base from diffusing into said oxidation rssistant coating.

Ref-memes Ciie in the file of this patent UNITED STATES PATENTS 2,536,74 Samuel Ian. 2, 1951 2,763,921 Turner Sspt. 25, 1956 Wainar Dec. 26, Gibson Oct. 8, Samuel 12111. 19, Wroinowski Mar. 29, Hill Feb. 28,

FOREEGN PATENTS Grea Britain Dec. 18, Grseat Briiain Sept. 23, 

6. A METALLIC STRUCTURE COMPRISING, IN COMBINATION, A BASE COMPRISING ONE OF THE METALS SELECTED FROM THE GROUP CONSISTING OF MOLYBDENUM AND MOLYBDENUM ALLOYS, AN INTERMEDIATE DIFFUSION BARRIER LAYER CONSISTING ESSENTIALLY OF CHROMIUM APPLIED TO SAID BASE BY THE DIFUSION PROCESS, AND AN OXIDATION RESISTANT COATING CONSISTING SUBSTANTIALLY OF MOLYBDENUM DISILICIDE APPLIED ON SAID INTERMEDIATE BARRIER LAYER, SAID BARRIER LAYER BEING OF SUFFICIENT THICKNESS TO PREVENT THE MOLYBDENUM OF SAID BASE FROM DIFFUSING INTO SAID OXIDATION RESISTANT COATING. 